Just click on the "Start Free Trial" link on the front page. It'll step you through the process of signing up and getting your subscription started.
If you go to a CrossFit Affiliate but they don't utilize BTWB, you can still specify your account association. Upon signing up, you'll be able to enter the name of your gym. If this gym is unclaimed (your gym hasn't setup with BTWB), you'll be associated to an Unclaimed version of your gym's page. The first 3 users to join an unclaimed gym will be the "moderators". You'll be able to utilize the Plan feature for the gym page so that way, if other members of your gym also join the unclaimed gym, you'll all be able to see the same workout if someone plans it so you all can log your results and appear on the leaderboards together.
If an affiliate owner then wants to come in and Claim the gym and make it an official Gym page, they'll be able to by following the Gym Sign-Up instructions.